Magento2.1, all attribute load via UI_Component. So you need to assign as a form element following way:
Create Vendor/Module/view/adminhtml/ui_component/category_form.xml
<?xml version="1.0" encoding="UTF-8"?>
<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
<fieldset name="display_settings">
<field name="customattribute">
<argument name="data" xsi:type="array">
<item name="config" xsi:type="array">
<item name="class" xsi:type="string">Magento\Catalog\Ui\Component\Category\Form\Element\Wysiwyg</item>
<item name="formElement" xsi:type="string">wysiwyg</item>
<item name="wysiwygConfigData" xsi:type="array">
<item name="settings" xsi:type="array">
<item name="theme_advanced_buttons1" xsi:type="string">bold,italic,|,justifyleft,justifycenter,justifyright,|,fontselect,fontsizeselect,|,forecolor,backcolor,|,link,unlink,image,|,bullist,numlist,|,code</item>
<item name="theme_advanced_buttons2" xsi:type="boolean">false</item>
<item name="theme_advanced_buttons3" xsi:type="boolean">false</item>
<item name="theme_advanced_buttons4" xsi:type="boolean">false</item>
<item name="theme_advanced_statusbar_location" xsi:type="boolean">false</item>
</item>
<item name="files_browser_window_url" xsi:type="boolean">false</item>
<item name="height" xsi:type="string">100px</item>
<item name="toggle_button" xsi:type="boolean">false</item>
<item name="add_variables" xsi:type="boolean">false</item>
<item name="add_widgets" xsi:type="boolean">false</item>
<item name="add_images" xsi:type="boolean">false</item>
</item>
<item name="template" xsi:type="string">ui/form/field</item>
<item name="source" xsi:type="string">category</item>
<item name="wysiwyg" xsi:type="boolean">true</item>
<item name="dataScope" xsi:type="string">customattribute</item>
<item name="sortOrder" xsi:type="number">50</item>
<item name="rows" xsi:type="number">8</item>
<item name="label" xsi:type="string" translate="true">Custom Group</item>
</item>
</argument>
</field>
</fieldset>
</form>
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